The present invention pertains generally to a method and apparatus for fabricating patterns on large optics. More particularly, the present invention pertains to a method and apparatus for fabricating patterns on large optics utilizing contact lithographic fabrication techniques.
Prior art techniques for fabricating patterns such as holographic optical elements on mirrors are represented by the two-beam interference approach. In this technique the substrate of a large optic, coated with metal, is coated with a photoresist material which is exposed to a set of fringes. Upon developing and etching, a holographic pattern is produced in the metallic layer.
The prior art has several drawbacks including requiring long pathlengths, sources having long coherence lengths, long exposure times, wavelength scaling, correction optics, fringe stabilization schemes and limited size and placement of HOES.
To overcome these and other disadvantages of the prior art, the present invention contemplates providing a device which can etch a pattern on a large substrate with accurate placement, accurate line spacing and which is readily scalable to much larger sizes. The pattern is transferred at the exact operating wavelength through the use of photolithographic techniques, thus eliminating the need for correction optics, such as null correctors, to correct for wavelength scaling aberrations. The pattern generated can be a single full aperture zone plate or several subaperture elements. The actual pattern generation is accomplished by a computer generated hologram (CGH) technique using an e-beam generated master and a flexible mask, thus eliminating the conventional two beam interference approach.
While techniques known in the art of photolithography are suitable to conventional applications, they are not easily applied to the fabrication of patterns on large optics.
Accordingly, we have added to the art of photolithography by providing an apparatus and method for depositing a layer of photoresist material in a relatively small localized area on a large curved optic. We have done so without the need for spin-coating, as is common in the prior art, and without the need of an oven to accomplish prebaking of the photoresist layer. We have also eliminated the need for an oven for post-baking of the photoresist layer after pattern generation using a suitable mask.
In addition we have eliminated the need for dipping of the coated substrate to develop the photoresist layer or etch the metallic layer. This has been done through the provision of a movable fluid-containment device which can deliver developing or etching fluid to a relatively small localized area on the large optic.